1. Field of the Invention
The invention relates to a method of transmitting operational signals to a receiver, the operational signals representing operations to be performed by the receiver, and including first operational data for receivers capable of performing first operations, and further operational data for receivers capable of performing further operations. The invention also relates to a method and arrangement for receiving and processing said operational signals.
2. Description of the Related Art
An example of a known method of transmitting operational signals as defined in the opening paragraph is the transmission of "level 2 or higher" teletext in accordance with the well-known World System Teletext standard. Briefly summarized, this teletext transmission allows teletext pages to be displayed in a display format that does not suffer from the restrictions of the basic "level 1" format. For example, whereas level-1 teletext transmissions allow the color of text on screen to be changed in spaces between words only, level-2 transmissions can assign different colors to individual characters of a word if, of course, the relevant receiver has the capability of displaying pages in the higher level format.
Enhancing an existing service such as teletext imposes a compatibility problem. In the above prior art, backwards compatibility is achieved in that the broadcasters compose a sensible level-1 page first, and then extend said page with further operational data defining the enhancements in accordance with the level-2 format and being ignored by level-1 receivers.
The invention is not restricted to teletext transmissions. It is envisaged that other emerging data services will increasingly suffer from the above-mentioned compatibility problem. A particular example of a service to which the invention applies is interactive television program broadcasting. In interactive television, operational signals are transmitted along with the television program which allow a user to submit his opinion about the program contents to the broadcaster, or even influence the course of the program. For example, the user may press a certain key of his remote control unit to express which one of the candidates Debbie, Anne or Laurie, is to win the Miss World contest which is being broadcast. In response to the user's choice, a receiver having a backwards channel performs a corresponding operation, for example, dialling a first telephone number for Debbie, a second telephone number for Anne and a third telephone number for Laurie. While the feature of calling a predetermined telephone number may be an established capability of existing receivers, newer receivers may have enhanced capabilities which the broadcaster may like to exploit.